These illustrations represent some of the steps and techniques used to produce
“pretty pictures” from HST images. Each thumbnail image links to a web-sized
JPEG and the PDF icon links to an Acrobat PDF document.

Bright, sharp spots and streaks from cosmic rays appear on every HST
exposure. Multiple exposures at the same pointing allow the randomly-appearing
noise to be removed relatively easily, as long as nothing else changed
in the meantime (easier for non-variable deep space targets such as nebulae
and galaxies rather than closer, fast-moving solar-system targets).

“Good” pixels from one exposure replace “bad” (cosmic ray)
pixels from another exposure in a combined image. Otherwise, the exposures
add together exactly as if it were one longer exposure.

Automatically setting the smallest image value to be black and the highest
value to white can force important areas of the image to be rendered invisible.
By selecting a minimum and maximum data value to render as black and white,
repsectively, (clipping) the significant regions of the image can be made
visible.

Clipping the bright end to a lower pixel value shows some detail, but
fainter details still may not visible. Clipping to still lower levels
brings out fainter details but saturates (forces larger areas
to be white) the bright regions.

Applying a non-linear (log, square root, etc.) transformation can compress
the dynamic range so that more detail becomes visible. Fainter details
can become visible without saturating the brightest regions.

Histograms are plots of the numbers of pixels at each brightness value,
showing the relative distribution of intensities. They can be useful to
guide the selection of clipping and intensity scaling. A flatter histogram
reflects more detail visible in all intensity levels. Intense peaks indicate
a concentration of values at a particular brightness.

Separate images are exposed through different color filters. The resulting
black and white or “grayscale” images are assigned a color that can be
viewed or reproduced (that may or may not be the color of the filter used
for the exposure). The separate images are combined in color image “channels”
that together produce a full range of hues.

Using filters that match the eye’s response can result in a natural or
“true” color image. Otherwise, the colors are enhanced or shifted from
what we would be able to see. In most cases, though, the colors in the
picture represent actual colors in the observed object. That is, what
looks red is redder in the object, but perhaps not as red as it appears.

In this case, four filters were used, with three assigned the additive
primaries: red, green and blue and the fourth assigned violet. In principle,
any hue may be assigned to any filter layer, but the maximum color range
and contrast usually is available by assigning the primaries. Other hues
are combination of the primaries so will generally desaturate the colors
unless the structure that appears in that filter is different from the
structure appearing in the other filters (as different hues in other layers).

HST’s WFPC2 instrument contains four CCD detectors (“chips”), each producing
an image 800x800 pixels. Three “wide-field” (WF) chips have a field of
view of 2.5 arcminutes. The fourth “planetary camera” (PC) chip sees an
adjacent, narrower field of 35 arcseconds.

Because of the instruments optical path, the resulting images are
rotated with respect to each other. The separate images must be rotated
and scaled to the same pixel scale before being combined into a mosaic.

The fields overlap somewhat but “seams” remain in the resulting mosaic
images.

Hubbles new Advanced Camera for Surveys (ACS) does not pose this
particular challenge. The camera does consist of two separate CCD detectors
(each 2048x4096 pixels in size), separated by a narrow gap, but they are
the same pixel size and scale. Most observations consist of overlapping,
offset exposures which can be combined to fill in the gap.

The relatively narrow field of view of the WFPC2 camera means that many
targets overfill a single image. Several carefully aimed exposures (“pointings”)
may be used to cover the field but must be combined afterward.

Overlapping areas can be cut around (masked) or combined.
The resulting images retain the resolution of the individual WFPC2 fields.

All
of the above illustrations together

These
and some other Illustrations showing specific Adobe Photoshop tools.